Reference is first made to gravure printing ink. In general printing, the quality of printing depends on the printability and printing effect of printing ink. The printability is a set of properties associated with the behavior of ink from transfer of ink on a printing machine to an object to be printed to completion of a printed surface, the properties mainly including fluidity, interfacial compatibility and drying of ink. On the other hand, the printing effect is the finish of printing that the ink having formed a printed surface develops, evaluated in terms of such properties as color tone, gloss, water resistance, and blocking resistance. Ink is basically composed of a coloring matter (dye or pigment), vehicle, and auxiliary agents (flow adjusting agents, drying adjusting agents, etc.). Of these, the vehicle has the most impact on the printability and printing effect. The vehicle is basically composed of a binder, wax, solvent and the like. By a choice of the binder, the nature of the vehicle is substantially changed, on which the printability and printing effect largely depend.
The binders are generally classified into solvent binders and aqueous binders. The solvent binders include urethane resins as proposed in JP-A H09-12956 (Patent Document 1). Nowadays, however, the considerations of air pollution, fire risk and working hygiene turn attention to the aqueous binders not using organic solvents.
The aqueous binders are generally divided into alkali soluble resins as typified by styrene-acrylic resins, water-soluble resins as typified by polyvinyl alcohol, and water-dispersed resins as typified by urethane emulsions (Patent Document 2: JP-A H02-238015) and acrylic emulsions.
In general, in the case of printing using an aqueous ink comprising a water-dispersed resin as the binder, the ink is applied from a printer onto an area to be printed at room temperature, and thereafter dried at or above the glass transition temperature of the water-dispersed resin until finished. When heated at or above the glass transition temperature, overall resin particles fuse together to form a uniform film which develops and improves water resistance. With an attention paid to drying on the printer among printability factors, the printing ink must maintain fluidity on the printer for a time as long as possible without drying. In this regard, the solvent type resin, alkali-soluble resin and water-soluble resin are non-problematic in that even if the resin once becomes a dry film, the resin resumes original fluidity when dissolved in solvent, alkaline aqueous solution or water, whereas the water-dispersed resin is problematic in that once the resin is dry so that resin particles fuse together at their surface, the resin does not resume fluidity even when contacted with water, leading to a failure of printing. On the other hand, the alkali-soluble resin suffers from poor alkali resistance and the water-soluble resin suffers from poor water resistance. Then conventional binders are sometimes prepared by combining the alkali-soluble resin or water-soluble resin with the water-dispersed resin. In the state-of-the-art, an attempt to improve printability encounters a loss of alkali or water resistance while an attempt to improve alkali or water resistance leads to insufficient printability.
To solve the problem, JP-A H10-176132 (Patent Document 3) proposes a binder for aqueous ink comprising a monomer selected from vinyl chloride, an alkyl (meth)acrylate having an alkyl moiety of 1 to 18 carbon atoms, and a monoalkenylbenzene, an ethylenically unsaturated monomer having a functional group, and another ethylenically unsaturated monomer. However, problems remain with respect to humidity resistance and gloss.
Also fluoroplastics are proposed in Patent Document 4: JP-A H11-35866. Although fluoroplastics are excellent in many aspects including weather resistance and corrosion resistance, a cost problem is inevitably left.
Reference is now made to inkjet ink. The inkjet recording system is excellent in quiet, recording speed, setting quality and running cost, and is generally on widespread use. For use in this system, an aqueous ink is selected from the aspects of ink physical properties, safety, and ease of handling. Ink compositions having a water-soluble dye such as an acidic dye, direct dye or basic dye, dissolved in a glycol solvent and water are known from Patent Documents 5 to 7: JP-A S53-61412, JP-A S54-89811, and JP-A S55-65269. They have the drawback of poor water resistance.
Thus the use of pigments as the coloring component is under investigation. Acrylic resins or styrene-acrylic resins having carboxyl groups incorporated therein (Patent Document 8: JP-A S61-235478) and aqueous resins in the form of ternary copolymers of acrylic acid, styrene and alpha-methylstyrene (Patent Document 9: JP-A H08-253716) are used to disperse pigments. Although improvements in water resistance and weather resistance are noticeable, there is left a room for improvement in dispersion (change with time) of pigments.
Next, reference is made to the receiving layer of recording sheets, especially of inkjet recording sheets. Currently the recording system using aqueous ink is often employed in the printer in the business machine system including personal computers, and accordingly the demand for recording material suited for that system is increasing. Meanwhile, a recording material having better characteristics is required in order to achieve further improvements in quality, colorfulness, appearance and definition of printed images. The recording material is used in diversified applications while requirements of properties such as water resistance and color development become increasingly stricter. As a typical receiving layer, there are proposed a recording sheet in which amorphous silica and a polymeric binder such as polyvinyl alcohol are combined and coated on a base paper (Patent Document 10: JP-A S55-51583) and a recording sheet comprising a base paper provided on front and back surfaces with an ink receiving layer containing porous pigment particles (Patent Document 11: JP-A S56-148584). Although a significant improvement in colorfulness or sharpness is achieved as compared with conventional recording sheets using wood-free paper, these receiving layers have the problems that a reduced coating weight allows for irregular spread of ink, resulting in printed images with substantial bleeding and that with an increased coating weight, bleeding is reduced, but the coating layer tends to spall off, causing dusting.
Patent Document 12: JP-A S62-288076 proposes a recording sheet in which binding strength is improved using a water-insoluble resin obtained from reaction of polyvinyl alcohol with acrylic acid and methyl methacrylate. Since this water-insoluble resin is anionic, fixation of an aqueous ink which is also anionic is adversely affected, failing to provide a solution capable of preventing ink bleeding or improving water resistance.
Further proposed are a recording sheet comprising a polycation polymeric electrolyte (Patent Document 13: JP-A S56-84992), a recording sheet comprising an ink receiving layer composed mainly of an ink-adsorbing cationic polymer binder (Patent Document 14: JP-A S58-24493), and a recording sheet comprising a cationic acrylic resin emulsion (Patent Documents 15 and 16: JP-A H11-123867 and JP-A 2001-199152). These proposals are not necessarily satisfactory with respect to color development and water resistance.
The receiving layers on recording sheets include those of the thermal transfer recording system. Among others, the dye diffusion transfer recording system is deliberately considered as the process capable of forming color hard copies having the closest image quality to the image quality of the silver salt photography. It is proposed to use a vinyl chloride resin in such receiving layer (Patent Document 17: JP-A 2008-30450). Since a low molecular weight emulsifier is used, the receiving layer is not necessarily satisfactory with respect to dye oozing and humidity resistance.
Further, the inventors proposed in Patent Document 18: PCT/JP2010/59418 a vinyl chloride resin using an acrylate oligomer. Since the polymerization ratio of vinyl chloride unit in polymer is low, there is left a room for improvement in parting property.